The mutagenic activity of ethylmethane sulfonate (EMS) as a function of its DNA alkylating ability has been studied in Salmonella typhimurium. The mutagenic activity of EMS in the base-pair substitution strain G46 and its repair deficient derivatives (TA1950, [uvrB]; TA92, [pKM101]; TA2410, [uvrB, pKM101] were compared. Ethylation levels were equivalent in wild type and uvrB cells, but the efficiency of induction of mutations (mutants/adduct) was different between the two cell types. EMS and other ethylating agents induce higher levels of mutation in wild type cells at low doses; at high doses the uvrB- cells show greater incidences of mutation. This phenomenon is being studied with other classes of alkylating agents and in other microbial strains.